The aims of this project are to develop mechanically passive prosthetic ankle-foot prototypes that can automatically adapt to different surface slopes on every step and to test the function of veterans with unilateral transtibial amputations when using the prototypes on an inclined instrumented treadmill. Two prototypes will be designed, manufactured, and tested within the two year time frame of the pilot study to determine the feasibility of this new device. The unique features of the design allow it to change its effective alignment for different walking slopes while also providing a neutral or slightly dorsiflexed ankle position for the swing phase of walking. Up to three veterans with unilateral transtibial amputations will participate in the study. The subjects will receive training on use of the prototype by a physical therapist that specializes in care of persons with amputations and will walk on a variety of slopes to become accustomed to the device. After subjects are accustomed to the device, they will walk with it on a split-belt instrumented treadmill set to different slopes (-10, -5, 0, 5, and 10 degrees) while ground reaction forces are measured. An eight-camera motion analysis system will also be used to measure the ankle motion during walking on the different slopes. Ankle torque versus ankle angle characteristics will be examined to determine if the ankle is adapting as expected. We will also measure perceived exertion and socket comfort when walking on the different sloped surfaces and expect the adaptable ankle-foot prosthesis to provide improvements on both measures compared to subjects' normal non-adaptable ankle-foot prostheses.